1. Technical Field
The present invention relates to an optical scanning device, to a method of controlling an optical scanning device, and to an image forming apparatus. More specifically, the present invention relates to an optical scanning device which scans laser light modulated according to image signals so as to display images.
2. Related Art
In an image display apparatus which scans laser light so as to display images, an optical scanning device for scanning laser light is used. The optical scanning device scans laser light modulated according to image signals in a two-dimensional direction. The image display apparatus causes laser light from the optical scanning device to be incident on a screen or the like so as to display images. In order to modulate laser light, in general, a pulse width modulation (hereinafter, referred to as ‘PWM’) method, in which a pulse width for turning on laser light is changed according to image signals, is used. In one frame of an image, in order to express grayscale levels according to image signals for all pixels, the pulse width of the minimum unit needs to be significantly small. As the number of pixels of an image is increased or as the number of grayscale levels of an image is increased, the minimum unit of the pulse width becomes smaller. In a high output laser light source, it is very difficult to perform switching according to the small pulse width accurately and at high speed. Accordingly, when it is difficult to increase a modulation frequency, scanning may be shared by using a plurality of laser light sources. For example, when the optical scanning device scans laser light in horizontal and vertical directions of a region to be irradiated, if a frequency for scanning laser light is higher in the horizontal direction than in the vertical direction, scanning may be shared by arranging the plurality of laser light sources in the vertical direction in a row. When the plurality of laser light sources are arranged in a row in the vertical direction, it is possible to display an image at a low modulation frequency, as compared with a case in which a single laser light component is scanned. In this case, however, a large gap occurs between traces of a group of spots in the region to be irradiated. As the number of laser light sources is increased, the gap becomes larger. Further, scanning in the horizontal direction is misaligned from the horizontal direction of the screen to be inclined. If the gap between scanning traces of the laser light components is conspicuous, it is difficult to obtain high-quality images. As a unit for scanning laser light so as not to cause the conspicuous gap, as many light sources as scanning lines may be arranged in one of the horizontal and vertical directions, and the laser light components may be individually scanned in only one direction. A technology which uses as many light sources as scanning lines has been suggested, for example, in JP-A-2003-21804.
However, when as many light sources as scanning lines are arranged, a great number of light sources need to be provided, and thus the optical scanning device comes expensive. Further, when a difference in intensity exists between the laser light components arranged in a row in the vertical direction or when a gap exists between the laser light components arranged in a row in the vertical direction, brightness irregularity occurs in images. When the difference in intensity exists between the laser light components, a light source unit may be adjusted in advance so as to supply the laser light components having the same intensity. In this case, an output of another laser light component is added to a laser light component having the minimum intensity, and thus brightness of laser light or grayscale level to be originally supplied is wasted. Further, when the gap exists between the laser light components, the laser light components may be moved and scanned so as to simply bury the gap or the laser light components may be arranged in a two-dimensionally direction in advance so as to bury the gap. In this case, brightness irregularity occurs between a region where the laser light components overlap and a region where the laser light components do not overlap. As such, in the related art, when scanning is shared by using a plurality of laser light components, it is difficult to display high-quality images.